Cross-polarisation times

In the case of the aromatic protonated carbons, cross-polarisation experiments can be performed. The t /2 cross-polarisation time (defined as the contact time required to produce half the maximum value of magnetisation that is possible by cross-polarisation) significantly increases with temperature, as shown in Fig. 17. 

The H- Si cross-polarisation time constants (Ts,h) associated with the Q species of both the 5% V-MCM and the standard MCM-41 calcined samples have been derived from variable-contact-time experiments. These values can be viewed as an indication of how fast the transfer of polarisation from protons to silicons occurs. The value for the 5% V-MCM sample is of the order of 0.15 ms, whereas for the standard sample it is 20 times greater, about 3 ms. 

Fig. 6.6.3. Dependence of intensity (at 188.29 MHz) on contact time for a sample of poly(trifluoroethylene). MAS rate 12.6 kHz. The plots can be fitted to a simple equation with effective Tip (incorporating both H and relaxation) of 14 ms and cross-polarisation times of 1.3 and 0.4 ms for the CF2 and CFH fluorines, respectively.

For resins synthesised under acidie conditions tertiary amides initially seen by CP/MAS were confirmed by CP/MAS. Under neutral or basic conditions the main constituents of the resin are A,A -dimethylolurea (102 ppm), monomethylolurea (102 and 78 ppm) and dimethylene ether linkages (90ppm). Using dipolar dephasing and cross-polarisation times it was possible to distinguish primary, secondary and tertiary substituted nitrogens. These results confirmed the existence of many moieties postulated by CP/MAS. 

In addition Duff et al. also undertook a quantitative analysis by utilising the large difference in cross-polarisation time for protonated and nonprotonated nitrogen. In a related study a series of resins were examined in which biuret linkages predominated [18]. These were formed by the reaction of formic acid and 4,4 -methylenebis(phenyl isocyanate) (MDI). The principal reactions that occur in the resins are shown in Fig. 15.2.16. 

So that the results from the delayed-contact and variable-contact data could be compared, they were normalized to give equal signal intensities at a contact time of 0.5 ms (zero delay in the delayed-contact experiment) after the variable-contact data had been adjusted to allow for the fact that full equilibration of proton and polarisation had not quite been reached after 0.5 ms. The difference in normalised signal intensity between the variable-contact and delayed-contact experiments, at a given time point t, is then a measure of the amount of 13C cross-polarising between 0.5 ms and t. 

Fig. 17 Cross-polarisation contact times, fi/2, for the aromatic carbons in the phenyl rings, in both the pure PET and the PET/TPDE blend O aromatic carbons in PET and aromatic carbons in the blend that contains 10% TPDE (from [12])...

Deconvolution of these peaks would yield a distribution of Q2 and Q3 sites as a function of temperature. However, cross-polarisation spectroscopy is not automatically a quantitative technique, since the signal s intensity depends -amongst others- upon the proximity of protons and upon the contact time, used in the experiment. 

A distinction between primary and secondary species can be made using 29Si CP MAS NMR. Primary species cause a chemical shift at -36 ppm (reference TMS), whereas secondary species cause a shift at -60 ppm. Following the procedures of Maciel and Sindorf,64,65 which take into account the dependence of the CP (Cross - Polarisation) line intensities with the CP contact time, the percentage of primary and secondary species is calculated. 

Figure 12.13 129Xe spectra of the blend of Figure 12.7 obtained with single pulse excitation (top spectrum) and with 1H-129Xe cross-polarisation at -30 C. The crosspolarisation time was 3 milliseconds and recycle time of 3 seconds. Cross-polarisation is effective only for Xe in the iPP matrix, the Xe atoms in the EP domains are too mobile. At higher temperatures also the CP resonance of Xe in iPP disappears... 

Cross Polarisation not only provides a better signal-to-noise ratio, but the experiment can also be done faster, because the repetition rate of the pulses is now determined by the proton relaxation times, which are a factor 10-100 smaller than those of the carbons. 

TCP, the cross polarisation relaxation time, is a measure of the strength of the dipolar contact between the 1H and the 13C nuclei. This strength depends both on the inter-nuclear distance and on the mobility of the chain or chain segments. The cross polarisation is less effective when the motion is fast, whereas rigid systems will cross-polarise rapidly. So a study of TcP will yield insight into the proximity of the nuclei and the mobility of the molecular chain or chain segments. 

This example is illustrated in Figure 13.9 in which the MAS spectrum (a) is represented with the spectrum obtained from cross polarisation (b). The comparison of the two clearly shows a relative increase in certain peaks these are the Si (OH) (Si)4, , n taking the values shown on the spectrum (b). The Si (OSi)4 signal is still present this is simply due to an extended contact time which permits the polarisation to be extended over 3 bonds. It may thus be necessary to vary the contact time in order to fully discriminate the atoms bearing hydroxyls, Quantitative analysis is not possible but the experiment can be used to determine the presence or absence of silanol groups. 

The dipolar coupling is also central to many schemes for magnetisation transfer between nuclei using double resonance techniques such as cross-polarisation. The data become more difficult to unravel when a nucleus experiences numerous different dipolar interactions at the same time. Even an increase to just three spins causes the lineshape... 

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